Tap changing transformers are commonly used to regulate AC voltage in both low power, low voltage applications, and high power applications at distribution level voltages. Distribution level regulators typically consist of a multi-tapped transformer winding coupled to a mechanical tap changer so that regulation within +/−10% of nominal voltage is possible. These tap changer designs incorporate various mechanisms to ensure that, when transitioning from one tap to the next under load conditions, load current is not interrupted and arcing and inter-tap short circuit current are minimized.
In low voltage applications (less than 1000 Volts-rms) and low power applications (less than 1 Million Volt Amps) mechanical tap changers are often implemented using a simpler design incorporating a sliding commutation brush which can be positioned at arbitrary points along an exposed transformer winding in order to achieve the change in effective turns ratio. This technique has much lower cost than a discrete tap changer of the type used at higher power levels, but does not provide the same performance and also requires more maintenance.
Electronic tap changers are also commonly used in low voltage and low power (less than 1,000 VA) to moderate power (approximately 500 k Volt Amp) levels. Now, referring now to FIGS. 1-3, three known devices are shown. In FIG. 1 an electronic tap changer 10 comprises an electronic switch 20, 22, 24 connected to each tap 12, 14, 16 of a multi-tapped transformer 40 or auto transformer. Typically, each switch 20, 22, 24 includes anti-parallel (back-to-back) connected silicon controlled rectifiers (SCRs) 30, due to their low cost, simplicity, and ruggedness. By actively selecting which SCRs 30 are firing (e.g., by using appropriate sensing and gating controls, for example), the effective turns ratio of the transformer 40 can be controlled, so that the output voltage may be varied for a constant input voltage (when applied as an AC voltage source 50), or, when applied as a voltage regulator, the output voltage may be maintained within a certain tolerance under conditions of varying input voltage. Tap changer 10 may include other components, as would be recognized by one of ordinary skill in the art, including for example, ground connections 32, loads 34, etc.
An alternative implementation to the basic electronic tap changer 10 of FIG. 1 is shown in FIG. 2. Here, a series transformer secondary winding 60 reduces the current through the electronic switches 20, 22, 24, while increasing the voltage applied to each switch.
In any SCR-based ‘on load tap changer’, provisions must be made to avoid both load current discontinuity and high inter-tap circulating current when commutating the load current from a switch that is conducting to another switch (i.e., making a tap change). This is the same fundamental problem which must be addressed in the design of high power, ‘discrete mechanical on-load’ tap changers. The unique problem in the case of SCR based tap changers is a result of the conduction characteristics of SCRs; an SCR may be turned on at any arbitrary time by applying a signal to its gate, but the SCR will cease to conduct only when the load current naturally falls to zero or reverses (normally once each electrical half cycle).
When commutating from the ‘present tap’ to a ‘new tap’, if the new tap SCR is fired before the present tap SCR has ceased conducting, the two SCRs will form a short circuit current path across the two transformer taps until the ‘present tap’ SCR current reverses. This short circuit current is potentially damaging to the SCRs and transformer windings, and, as the short circuit current flows through the source impedance and the transformer impedance, may cause a decrease in the regulator's output voltage. Conversely, if a delay is used such that the ‘present tap’ SCR is allowed sufficient time to turn off and regain its voltage blocking ability before the ‘new tap’ SCR is activated, inductive loads may cause damaging or unacceptable voltage transients in response to the current discontinuity which exists during the delay period.
Previous tap changers, as shown in FIG. 3, solved the problems identified above by adding a commutating current path 70 through an impedance element, for example, a commutation resistor 80, an inductor (not shown), or other current limiting device. This is a basic representation of one of many methods commonly utilized in high power, mechanical tap changers. In the device shown as FIG. 3, when commutating from tap 12 to tap 14, the anti-parallel SCR pair 26 connected to the commutation impedance 80 is first gated, resulting in current flow between the two taps, taps 12 and taps 14, which is limited by the impedance 80 to an acceptable level. After the tap 12 conducting SCR 20 has naturally ceased to conduct and following a delay sufficient to ensure that the SCR 20 has regained its voltage blocking capacity, the tap 14 SCR pair, SCR 22 may be fired with no concern for a current discontinuity as the load current will flow through the impedance 80 of tap 14 until the SCR 22 is fired, at which time the gate signals of the anti-parallel SCR pair 26 are removed.
The wiring scheme of FIG. 3, or one of its known derivatives, could be implemented on each tap in a 3-phase regulator in order to implement an acceptable commutation scheme for all possible tap changes. The additional complexity of this scheme, however, results in substantial additional cost which may render the entire device impractical, and the additional control complexity and parts count reduces the reliability of the device.